US 6869877 B2
A capacitor for a memory device is formed with a conductive oxide for a bottom electrode. The conductive oxide (RuOx) is deposited under low temperatures as an amorphous film. As a result, the film is conformally deposited over a three dimensional, folding structure. Furthermore, a subsequent polishing step is easily performed on the amorphous film, increasing wafer throughput. After deposition and polishing, the film is crystallized in a non-oxidizing ambient.
1. A method of forming a conductive oxide in an integrated circuit, comprising:
depositing an amorphous ruthenium oxide layer; and
crystallizing said amorphous ruthenium oxide layer by annealing in an ambient comprising animoriia gas in the absence of an oxidizing agent.
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16. A method of forming a conductive oxide in an integrated circuit, comprising:
depositing an amorphous ruthenium oxide layer;
patterning said amorphous ruthenium oxide layer; and
crystallizing said amorphous ruthenium oxide layer by annealing after said patterning.
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The present application is a divisional of U.S. application Ser. No. 09/546,492, filed Apr. 10, 2000 Pat. No. 6,492,241.
The present invention relates to integrated semiconductor memory cell capacitors. In particular, the present invention relates to a memory cell capacitor fabricated with conductive metal oxides and high dielectric constant materials and a method for manufacturing such a capacitor.
A memory cell in an integrated circuit, such as a dynamic random access memory (DRAM) array, typically comprises a charge storage capacitor (or cell capacitor) coupled to an access device, such as a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET). The MOSFET functions to either apply or remove charge on the capacitor, thus affecting a logical state defined by the stored charge. The amount of charge stored on the capacitor is proportional to the capacitance, C=kk0A/d, where k is the dielectric constant of the capacitor dielectric, k0 is the vacuum permittivity, A is the electrode area and d is the spacing between the electrodes.
As the packing density of memory cells continues to increase, each capacitor must still maintain a certain minimum charge storage to ensure reliable operation of the memory cell. It is thus increasingly important that capacitors achieve a high stored charge per footprint or unit of chip area occupied.
An integrated capacitor generally has a bottom electrode plate, or a storage electrode, and a top electrode plate, or a reference electrode, separated by a dielectric layer. Several techniques have recently been developed to increase the total charge capacity of the cell capacitor without significantly affecting the chip area occupied by the cell. These techniques include increasing the effective surface area of both the storage and reference electrodes by creating folding structures such as those in trench, stack or container capacitors. Such structures better utilize the available chip area by creating three-dimensional shapes to which the conductive electrodes and capacitor dielectric conform. The surface of the electrodes may be further increased by providing a roughened surface to the bottom electrode over which the capacitor dielectric and the top electrode are conformally deposited.
A container capacitor, for example, as shown in
In order to further increase the capacitance of the capacitors, other techniques concentrate on the use of new dielectric materials having a higher dielectric constant “k”, often referred to as high-k materials. Such materials include tantalum oxide (Ta2O5), barium strontium titanate (BST), strontium titanate (ST), barium titanate (BT), lead zirconium titanate (PZT) and strontium bismuth tantalate (SBT). The effective dielectric constants of these materials are significantly higher than conventional dielectrics (e.g., silicon oxides and nitrides). For example, the dielectric constant of silicon oxide is about 3.9, and the dielectric constant of the new materials can range from 20 to 40 for Ta2O5, up to 300 for BST; the dielectric constants of some materials can be even higher (600 to 800). Using such materials enables the creation of much smaller and simpler capacitor structures for a given stored charge requirement, enabling the packing density dictated by current and future circuit designs.
Difficulties have been encountered, however, in incorporating these materials into fabrication process flows. For example, Ta2O5 is deposited by chemical vapor deposition (CVD) employing organometallic precursors in a highly oxidizing ambient environment. After deposition, the material is typically annealed to remove carbon. This annealing process is typically conducted in the presence of oxidizing agents, such as oxygen (O2), ozone (O3) or nitrous oxide (N2O or NO), while volatile carbon complexes are driven out.
Due to the volatility of the reactants and by-products of processes for forming high k materials, surrounding materials are subject to degradation. For example, when the bottom electrode plate is made of metal or polycrystalline silicon (polysilicon), which is connected by a polysilicon or tungsten plug to the silicon substrate, all these materials can be oxidized during the deposition and anneal of the high k material. Although electrodes can be made of noble metals, such as platinum, where the noble metals are not easily oxidized, oxygen can still diffuse through the metal electrodes. Therefore, the surrounding oxidizable materials, including the polysilicon plug and the silicon substrate below, are still subject to degradation.
Oxidation of the electrode, the underlying polysilicon plug or the underlying substrate reduces conductivity of these electrical elements, while oxidation of electrode surfaces adjacent the dielectric reduces cell capacitance due to the formation of a layer of oxide with a relatively low dielectric constant. These problems have been viewed as major obstacles to incorporating high k materials into integrated circuits. Past efforts have therefore focused on using highly conductive diffusion barriers as the bottom electrode plate between the high dielectric material and the oxidizable elements, such as polysilicon plugs.
In order to solve the above problems in making a high-k capacitor, highly conductive metal oxides, such as ruthenium oxide (RuOx) and iridium oxide (IrOx) have been used to form the electrode plates. Such oxides are not corroded by oxidizing atmospheres, making them favorable candidates in avoiding the aforementioned electrode oxidation problem. At the same time, their barrier function can prevent the oxidation of underlying conductive plugs.
However, existing processes for fabricating RuOx/high-k container capacitors with the structure of
There is thus a need for a fabrication method of a high-k/metal oxide container capacitor with improved structure conformality and uniformity, which will increase the total capacitance while minimizing leakage of the capacitor.
One object of the current invention is to provide a new process of fabricating a bottom electrode for an integrated capacitor with a more conformal structure, thereby improving the uniformity and reliability of the capacitor, and also the process window to facilitate integration.
Another objective is to increase the etching efficiency during patterning of a conductive oxide film, particularly by polishing processes.
Another objective is to provide a structure of a container capacitor with a lower electrode plate having less sharp corners, to improve the reliability and decrease the leakage of the capacitor.
In order to achieve the above objectives, the preferred embodiments focus on an improved film deposition process and a film crystallization process for a conductive oxide film. Desirably, the process deposits a metal oxide film, in the preferred embodiment RuOx, under a relatively low temperature, such as will produce an amorphous film. As a result, the metal oxide film is more conformally deposited over a three-dimensional folding structure, uniformly covering the structure. Furthermore, since the amorphous metal oxide film is much softer compared to the crystallized film, subsequent patterning steps can be more efficient so as to increase the manufacturing efficiency and throughput.
In order to increase the conductivity of the metal oxide films, a post-deposition high temperature anneal crystallizes the metal oxide film, desirably in a non-oxidizing ambient. Preferably, this annealing process is conducted after the CMP polishing. As mentioned earlier, the metal oxide film is easier to etch, and particularly easier to polish, in an amorphous state. Moreover, the rim or edge area of the metal oxide film that serves as a bottom electrode plate of the capacitor is made less sharp due to a more conformal deposition. Consequently, the formation of more rounded corners of the bottom electrode plate reduces field strength and the risk of current leakage during operation of the circuit.
A high k capacitor dielectric layer is then formed on top of the bottom electrode plate, followed by formation of a top electrode plate. This top electrode plate, according to another aspect of the invention, is preferably made with the same conductive metal oxide as used for making the bottom electrode, and by a similar process. Accordingly, the top electrode plate is preferably also formed by depositing an amorphous film at a low temperature, and the film is later annealed to crystallize under a high temperature, non-oxidizing environment. The top electrode is preferably patterned (e.g., by conventional photolithography and dry etch) before the annealing step. In other arrangements, the top electrode can be crystalline as deposited, and can be formed of alternative materials.
Other aspects and advantages of the invention will be apparent from the Detailed Description below, and from the appended claims.
The present invention is described in more detail below in connection with the attached drawings, which are meant to illustrate and not to limit the invention, and in which:
Although the present invention is described in the context of fabricating memory devices, those skilled in the art of integrated capacitor fabrication will readily find application for the principles disclosed herein in many other contexts.
In order to understand the context in which the preferred embodiments are employed, reference is made to
As shown in
A conductive contact 108 is shown extending through the first insulating layer 107 to electrically contact an active area 102 between the transistors 101 and 103. In the preferred embodiment, the material of the conductive contact 108 comprises conductively doped polycrystalline silicon. Advantageously, polysilicon can be deposited with good step coverage into deep, narrow contact openings by chemical vapor deposition (CVD), polysilicon is compatible with the exposed substrate 100, and furthermore exhibits a high melting point to withstand further front-end high temperature processing. In accordance with industry terminology, the conductive contact 108 shall be referred to as a “poly plug”.
As shown in
A container capacitor 120 is then fabricated over the planarized surface 111. As shown in
An opening 112 is then formed in the structural layer 110, such as by using photolithography and etching techniques, exposing at least a contact portion of the underneath conductive plug 108. This opening 112 defines a container, in which a container capacitor is to be fabricated. The skilled artisan will readily appreciate other three dimensional folding structures to increase capacitor surface area, including trench capacitors, stacked stud configurations, finned structures, etc.
A barrier layer 313, typically comprising a metal nitride (e.g., TiN, TaN, WN, etc.) is first formed to intervene between the polysilicon plug 108 (
As also shown in
This low temperature deposition advantageously provides a conformal and uniformly thin film metal oxide layer 314, alleviating the complications and problems caused by the non-conformal film deposition. In particular, with an aspect ratio greater than about 3:1, the low temperature deposition produces a step coverage greater than about 50%, preferably greater than about 75%, and more preferably greater than about 90%. The skilled artisan will appreciate that “step coverage” is defined as a ratio of field coverage to sidewall coverage in the context of deposition over vias, as in the illustrated embodiment. In view of the present disclosure, the skilled artisan can readily determine appropriate conditions for amorphous deposition of conductive oxides using other chemistries or other pressure conditions.
With reference now to
Following the CMP, the resist or spin-on-glass remaining in the container is removed by either a wet etch or dry strip process. Then a high temperature annealing process is advantageously applied to the conductive oxide layer 314′, crystallizing the film to improve conductivity and thermal stability. In the illustrated embodiment, the metal oxide film 314′ is preferably annealed in a non-oxidizing environment, such as ammonium (NH3), nitrogen (N2), argon (Ar), helium (He) or other inert gases. Preferably, the anneal is conducted in a nitrogen-containing ambient. The annealing temperature is preferably between about 400° C. and 900° C., more preferably between about 500° C. and 850° C., and most preferably between about 700° C. and 800° C. This annealing process enhances conductivity of the metal oxide film 314′ and stabilizes its chemical structure.
Due to the fact that the annealed conductive metal oxide film 314′ serves as the bottom electrode plate of the container capacitor, there are some advantages for having the annealing or crystallization process after the CMP step. As a result of the CMP process, a sharp corner is formed at the rim area of the film 314′. Improved conformality due to the low temperature deposition of amorphous metal oxide as discussed above, however, somewhat reduces the sharpness of this corner. Consequently, the more rounded corner of the bottom electrode plate reduces operational field strength and attendant risk of current leakage.
It is well known in the art that tantalum oxide can be formed by chemical vapor deposition, using an organometallic precursor, followed by a high temperature anneal to drive out carbon complexes and cure the layer. Typically, the anneal comprises heating the substrate 100 (shown in
The metal oxide film 318 can be patterned, as shown in
Another advantage of using the conductive metal oxides for the top electrode material is that it serves as a high oxygen-content source, inhibiting oxygen diffusion from the underlying high k material 316. This oxygen maintenance is important for any type of high k capacitor, because even a very small amount of oxygen depletion from the high k material 34 can result in conductive charge leakage paths in the capacitor. Suitable conductive oxides include the ruthenium oxide (RuOx) of the illustrated embodiment, as well as iridium oxide (IrOx), and rhodium oxide (RhOx). Where the electrode does not form a volatile compund in the presence of an oxidizing ambient, such as with IrOx, an oxidizing ambient during the crystallizing anneal can be advantageous. Alternatively, since the deposition does not occur over an easily oxidized material, the top electrode can be deposited in crystalline form at higher temperatures.
The results displayed in
Although the foregoing invention has been described in terms of certain preferred embodiments, other embodiments will become apparent to those of ordinary skill in the art in view of the disclosure herein. Accordingly, the present invention is not intended to be limited by the recitation of preferred embodiments, but is intended to be defined solely by reference to the appended claims.